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Parnaíba (pərnīē´bə), river c.800 mi (1,290 km) long, rising in the highlands of NE Brazil. It flows generally north, forming the boundary between Maranhão and Piauí states, and enters the Atlantic Ocean through a delta near the town of Parnaíba, which is the shipping center for the valley; Teresina is the chief upstream city. Much of the river is navigable, although it is filled with rapids.